Maps a.k.a, associative array, map, or dictionary
Welcome message from author
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
Maps
a.k.a, associative array, map, or dictionary
Tecniche di programmazione
Definition
A.A. 2017/20182
! In computer science, an associative array, map, or dictionary is an abstract data type composed of (key, value) pairs, such that each key appears at most once
! Modern programming languages natively supports them E.g. Perl, Python, Ruby, Go
! Implemented through hash tables or tree data structure
V1[42] = “h2g2” V2[“h2g2”] = 42
Tecniche di programmazione
Java Collection Framework
A.A. 2017/20183
Tecniche di programmazione
Map interface
A.A. 2017/20184
! Map<K,V>! K: the type of keys maintained by this map! V: the type of mapped values
! Add/remove elements! value put(key, value)! value remove(key)
! Search! boolean containsKey(key)! boolean containsValue(value)
Tecniche di programmazione
Map interface (cont.)
A.A. 2017/20185
! Nested Class! Map.Entry<K,V>! A map entry (key-value pair).
! Set<Map.Entry<K,V>> entrySet()! Returns a Set view of the mappings contained in this map
! Set<K> keySet()! Returns a Set view of the keys contained in this map
! Collection<V> values()! Returns a Collection view of the values contained in this
map
Tecniche di programmazione
Map Family Tree
A.A. 2017/20186
Tecniche di programmazione
HashMap and Chaining
A.A. 2017/20187
0
m–1
U (universe of keys)
K (actual keys)
k1
k2
k3
k5
k4
k6
k7k8
K1,V1 K4,V4
K3,V3 K2,V2
K6,V6 K8,V8
K7,V7
Tecniche di programmazione
HashMap and Chaining
A.A. 2017/20188
! Non duplicated keys (values could be duplicated)! Chaining is not used to store multiple keys with the same value.
Each key should be unique! Chaining is used to solve the collision problem.
Tecniche di programmazione
HashMap
A.A. 2017/20189
! Non duplicated keys (values could be duplicated)! Not ordered (neither sorted)
! Implementation is based on a hash table! Operations put(k, v), get(k), remove(k), containsKey(k) have
complexity mostly O(1)
! Requires to override hashCode() equals()! Key object must be immutable
Tecniche di programmazione
HashMap vs HashSet
A.A. 2017/201810
! HashMap allows to insert key-value pairs. Each key is associated to a value
! HashSet allows to insert an object in a collection of object. The object itself (or part of it) is the key
! Similarties:! Do not accept duplicated key! Not ordered (neither sorted)! Implementation is based on a hash table! Requires to override hashCode() equals() for the Key object! Key object must me immutable (at least for the field used in
hashCode() and equals())
Tecniche di programmazione
HashMap complexity
A.A. 2017/201811
HashMapput(key, object) O(1)get(key) O(1)remove(key) O(1)containsKey(key) O(1)containsValue(object) O(N)
Tecniche di programmazione
HashMap complexity
A.A. 2017/201812
HashMapput(key, object) O(1)get(key) O(1)remove(key) O(1)containsKey(key) O(1)containsValue(object) O(N)
containsValue() will probably require time linear in the map size for most implementations of the Map interface – i.e. it is O(N)
Tecniche di programmazione
Collection Family Tree
A.A. 2017/201813
Tecniche di programmazione
LinkedHashMap
A.A. 2017/201814
! Implementation is based on a hash table and a double-linked list running through all of its entries:! Operations put(k, v), get(k), remove(k), containsKey(k) have
complexity mostly O(1)! Non duplicated keys
! Values could be duplicated! Ordered (usually insertion-order)
! Insertion order is not affected if a key is re-inserted
! Not sorted
Tecniche di programmazione
Licenza d’uso
A.A. 2017/201815
! Queste diapositive sono distribuite con licenza Creative Commons “Attribuzione - Non commerciale - Condividi allo stesso modo (CC BY-NC-SA)”
! Sei libero:! di riprodurre, distribuire, comunicare al pubblico, esporre in pubblico,
rappresentare, eseguire e recitare quest'opera! di modificare quest'opera
! Alle seguenti condizioni:! Attribuzione — Devi attribuire la paternità dell'opera agli autori
originali e in modo tale da non suggerire che essi avallino te o il modo in cui tu usi l'opera.
! Non commerciale — Non puoi usare quest'opera per fini commerciali. ! Condividi allo stesso modo — Se alteri o trasformi quest'opera, o se
la usi per crearne un'altra, puoi distribuire l'opera risultante solo con una licenza identica o equivalente a questa.
! http://creativecommons.org/licenses/by-nc-sa/3.0/
Related Documents
